Focusing Conditions in Neutron Inelastic Scattering Experiments with the IPEN Triple-Axis Spectrometer

摘要

The study of dynamic behaviour of the crystalline structure is one of the most important applications of neutron inelastic scattering technique. As part of the neutron inelastic scattering program of IPEN Nuclear Physics Division, a triple axis crystal neutron spectrometer is built and, in order to verify operational conditions and performance of the instrument, measurements of the known dispersion relations of copper are performed. In connection with these recent improvements, it deals with focusing conditions in coherent neutron inelastic process in which a single quantum of vibrational energy (phonon) is exchanged between the neutron and the crystal. For the instrument focusing conditions attainment mathematical expressions were deduced taking into account the finite collimation, incident and scattered wave vectors, and interplanar spacing of the monochromator and the analyser crystals. The focusing conditions were graphically represented in order to have a practical and adequate routine procedure for the focusing of the spectrometer. An experimental test is made by measuring acoustic phonon peaks propagating in the /00xi/ symmetry direction of a copper crystal. The measurements were performed in focusing conditions such as: optimum, good and defocused; the agreement between experimental results and theoretical predictions shows the usefulness of the adopted graphical method. (Atomindex citation 13:643786)